In recent years it has become commonplace for a subscriber to have various communication devices with which he can communicate, e.g. a home telephone, a cellular telephone, a facsimile line, etc. These devices may be connected to line circuits of different telephone switching systems.
There have been generally two ways of automatically establishing a calling link to these devices. The first way is to have separate telephone numbers for each device. In some such cases, line concentration can be utilized, e.g. in a key telephone system several key telephone sets are used, each having access to an equal or fewer number of lines. In order to make it appear that only a single telephone number is used, line equivalence for plural directory numbers is provided at the main switching office to which the key telephone system trunks are connected.
A second way is used to utilize a single directory number which rings plural devices connected in parallel to a single line, and after one of the devices answers the call, a specialized tone received from the line from a calling device indicates which specialized device is intended. Switching to that device can be then done either manually or automatically. This procedure is sometimes implemented with a facsimile machine connected to the same telephone line as a telephone set.
However none of the above systems allow different kinds of devices which can be connected to different line circuits within a switching system or in different switching systems, which are designated to a single subscriber, to be accessed using a single directory number, and wherein use of one device blocks out the use of the others.
Such systems also do not allow simultaneous ringing of a home telephone device and cellular telephone or other wireless device, using the same telephone number, to reach a particular subscriber, which would be highly desirable from the user's point of view.